1. Field of the Invention
This invention relates to a waste heat recovery boiler apparatus for generating steam by utilizing the heat of exhaust gases of gas turbine machines, and more particularly to a waste heat recovery boiler apparatus which is capable of lowering the concentration of oxides of nitrogen contained in the exhaust gases.
2. Description of the Prior Art
One type of waste heat recovery boiler apparatus of the prior art which is operatively connected to a gas turbine machine comprises a superheater, an evaporator, a fuel economizer and a stack arranged in the indicated order along a flow of exhaust gases from the gas turbine machine between its upstream end and its downstream end. Steam is generated in the superheater and introduced into a steam turbine, for example.
As is well known, a gas turbine machine generally comprises a compressor for compressing introduced air, a combustor for burning mixtures of the pressurized air and fuel, a turbine driven by combustion gas produced by the combustion of the fuel-air mixture, a generator connected to a load, and a system for injecting water or steam for lowering the concentration of oxides of nitrogen in exhaust gases.
In a combined cycle power plant constructed as aforesaid, it has hitherto been customary to lower the concentration of oxides of nitrogen contained in exhaust gases of the turbine machine by injecting water or steam into the combustor through a system for injecting water or steam, in an attempt to avoid the problem of air pollution by the noxious components of the exhaust gases released into the atmosphere from the gas turbine machine. However, this solution has a disadvantage in that the thermal efficiency of the combined cycle power plant shows a linear decrease as the ratio of the quantity of water or steam injected into the combustor to the quantity of air introduced into the combustor increases. For example, if the ratio of the quantity of water or steam injected into the combustor to the quantity of air introduced into the combustor is 1 percent, the thermal efficiency of the combined cycle power plant decreases 3 percent; if the ratio is 2 percent, the thermal efficiency decreases 6 percent. Thus an increase in the quantity of water or steam injected into the combustor which is necessary to satisfy the requirements to maintain the concentration of oxides of nitrogen in exhaust gases at a level which is considered acceptable for the population will cause a reduction in the thermal efficiency of a gas turbine machine and a combined cycle power plant. Moreover, if a system is to be established for limitations on the total volume of oxides of nitrogen released into the atmosphere and the oxides of nitrogen released into the atmosphere is to be limited to the level of less than 10 to 20 ppm, it will no longer be possible to satisfy the formulated requirements by means of injection of water or steam into the combustor.
To cope with this situation, proposals may be made to remove the oxides of nitrogen from the exhaust gas, in addition to said technique in which water or steam is injected into the combustor to decrease the amount of oxides of nitrogen in the exhaust gas. A dry catalytic reduction process which is one of the denitration processes consists in injecting ammonia into exhaust gases and causing the exhaust gases treated to pass through a reactor charged with a catalyst containing iron oxide so that oxides of nitrogen can be decomposed by reduction into nitrogen (which is innocuous) and steam. It is known that, in this denitration process, denitration efficiency depends primarily on the catalyst layer reaction temperature or the temperature of the exhaust gases passing through the catalyst layers of the denitration device. More specifically, denitration efficiency abruptly increases if the reaction temperature rises to the range between 200.degree. and 300.degree. C, with denitration efficiency being substantially maximized when the reaction temperature exceeds 300.degree. C.
The temperature of exhaust gases of a gas turbine machine may vary depending on the output power of the gas turbine machine. For example, in case the output power of the gas turbine machine is approximately 60 MW, exhaust gases of the turbine machine will have a temperature of about 550.degree. C. The heat of the exhaust gases is recovered as the stream of exhaust gases successively passes through the superheater, evaporator, and fuel economizer of a waste heat recovery boiler apparatus. Thus, when the exhaust gases are released into the atmosphere, the temperature thereof is reduced to about 200.degree. C. Conversely, feed water to the boiler is heated and converted into steam of about 450.degree. C. Therefore, even if the denitration device is installed at the exit end of the waste heat recovery boiler apparatus, the denitration device will function with a low degree of efficiency as is clear from the aforesaid temperature characteristic of the denitration device, because the temperature of the exhaust gases is about 200.degree. C. This will make it impossible to reduce the quantity of oxides of nitrogen contained in the exhaust gases to a desired level. If the denitration device is installed at the entrance end of the waste gas recovery boiler apparatus, problems will be encountered with regard to the high temperature proof of the reactor with a catalyst and a lowering in the efficiency of the catalyst, because the exhaust gases have an elevated temperature of over 500.degree. C.